1. Field of the Invention
The present invention relates to a method for refining oils, particularly domestic edible oils and petroleum crude oils, using non-aqueous membrane separation. More particularly, the present invention relates to a method for refining oils by membrane separation technology using a polyetherimide membrane.
During the past decade, considerable interest has been focused upon the processing of oils, particularly edible oils such as soybean oil, cotton seed oil, corn oil and the like. In the processing of such oils, treating the oil with an organic solvent, such as hexane, has been common to obtain micella. Following such treatment, the organic solvent is removed to yield a crude oil composition. Unfortunately, the crude oil so obtained typically contain up to 10%, by weight, of impurities including phospholipids, organic sulfur compounds, waxes, dye compounds and the like. These impurities typically have adverse effects and removing them from the crude oil to enhance the usefulness and marketability of the product is necessary.
Workers in the art have discovered that these impurities could be effectively removed from crude oil by means of different types of industrial membranes, selection of a particular membrane being dependent upon the materials to be separated. Thus, for example, micro-filtration involves the membrane separation of macro molecules ranging from 500 to two million Angstrom units, ultrafiltration with molecules ranging in size from 40-2,000 Angstroms and reverse osmosis with ions and molecules ranging in size up to 20 Angstroms.
For example, in an ultrafiltration/reverse osmosis process, a crude glyceride oil composition is diluted with an organic solvent such as hexane and contacted with an ultrafiltration reverse osmosis membrane. Such membranes are typically polysulfones, polyacrylonitriles, or various polyamides. During the process the solvent is removed to yield a membrane permeable solution containing a degummed oil. Although, such techniques have met with a certain degree of success, inherent limitations have precluded the total exploitation thereof.
More specifically, the complete removal of phospholipids from the crude oil is impeded by the characteristics of the membrane employed. This has made it necessary to use many processing steps including, for example, degumming, refining, bleaching and deodorizing to remove free fatty acids, phosphatides, particulate, chlorine materials and the like. Considerable amounts of energy in the form of steam or electricity are required in affecting these processes, each of which is required to obtain properly processed crude oil. Accordingly, workers in the art have focused their interest upon the development of alternate methods for refining such oils.
2. Description of the Prior Art
Possibly relevant prior art references are:
U.S. Pat. No. 2,930,754 describes a method for separating hydrocarbons, e.g., aromatic and/or olefin from gasoline boiling range mixtures, by the selective permeation of the hydrocarbon through certain cellulose ester non-porous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons, i.e., aromatic, unsaturated, saturated, by passing some of a mixture thereof through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.)
U.S. Pat. No. 3,370,102 describes a process for separating a feed into a permeate stream and a retentate stream and uses a sweep liquid to remove the permeate from the face of the membrane to maintain the concentration gradient driving force. The process can be used to separate a variety of mixtures including various petroleum fractions, naphtha, oils and hydrocarbon mixtures.
U.S. Pat. No. 3,847,818 to Madsen et al describes devices that are generically suited for separating liquids into two fractions by means of a semipermeable membrane. The only description of the fluid passing through the device is it is a "liquid". The plurality of retentates of Madsen et al are removed at the top and at the bottom through passage 17. These retentate streams are not processed again.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to separate aromatics from saturates via pervaporation.
U.S. Pat. No. 4,366,062 teaches reverse osmosis using a composite isocyanurate membrane. The method selectively separates at least one water soluble material from an aqueous solution. The membrane comprises a microporous substrate and a barrier layer about 0.01 to 0.1 micron thick. It is composed of a cross-linked polymeric material having an isocyanurate structure and substituents appended thereto selected from among hydrogen, glycidyl groups and alkyl radical groups having from two to five carbon atoms that may also contain functional hydroxyl groups or glycidyl groups. The crosslinked polymeric material has ester or ether linkages or combinations thereof connecting the isocyanurate structures to each other.
U.S. Pat. No. 4,557,949 teaches a method for making the reverse osmosis semipermeable membrane shown in U.S. Pat. No. 4,366,062.
U.S. Pat. No. 4,787,981 to Tanashi et al describes a process for the purification of crude glyceride oil compositions containing gums, waxes and organic solvents, by contacting the diluted crude oil composition with a semipermeable membrane comprising a polyimide. The resultant permeate is then bleached with clay and deodorized to obtain a purified glyceride oil.
U.S. Pat. No. 4,929,357 to Shucker describes the use of a non-porous isocyanurate crosslinked polyurethane membrane for separating aromatic-from non aromatic hydrocarbons.
U.S. Pat. No. 4,929,358 to Koenitzer describes the separation of aromatic hydrocarbons from non aromatics by permeation of the aromatic compound through a polyurethane-imide membrane under pervaporation or reverse osmosis conditions.
European Application 0044872 teaches selectively separating water soluble materials from a solution under reverse osmosis conditions using a membrane having a porous support layer carrying a barrier layer of cross-linked isocyanurate polymer.
Japanese Application 81/160960 teaches an isocyanurate network terpolymer useful for the production of a selective permeation membrane. A polymer having hydroxyl groups and tertiary amine groups in the side chain is reacted with cyanuric chloride and subjected to terpolymerization by reacting the tertiary amine groups with the resultant hydrochloride to give a polymerized polyisocyanurate. A polymer made using glycidyl methacrylate-styrene copolymer, diethyl amine in benzene and methanol was produced having a 2-hydroxy-3-diethylaminopropyl group. This (polytetra fluorethylene) polymer was crosslinked with cyanuric chloride and cast on a PTFE plate and kept for 24 hours at 40.degree. to give a 44.mu. membrane. This membrane was used to separate a mixture of cyclohexane and benzene under pervaporation conditions. A permeate gas that was 100% benzene was recovered.